The record figures include 12 people arrested for the Westminster terror attack last March.
Photograph: Alberto/Pacific/Barcroft Images

The number of people arrested for terrorism-related offences in Britain rose by 58% to a record high of 412 in 2017 – one of the most intense periods of terrorist attacks in recent history.

The Home Office quarterly statistics published on Thursday show there were 412 arrests in 2017 compared with 261 in 216.

The record figures include 12 people arrested for the Westminster attacks in March, 23 people linked to the attack in Manchester in May, 21 arrests connected with the London Bridge attack in June and one arrest in connection with the Finsbury Park mosque attack later that month. A further seven arrests were made in connection with the attack on Parsons Green tube station in September.

The figures reflect the growing threat from jihadists in Britain. The director general of MI5, Andrew Parker, spoke in October of “a dramatic upshift in the threat this year” to the “highest tempo I’ve seen in my 34-year career”.

The detailed Home Office figures show that the 412 arrests in 2017 resulted in 135 people being charged – 110 of them for terrorism-related offences. More than half – 228 or 55% – were released without charge, 33 were released on bail pending further investigation and 13 faced alternative action.

So far, 29 of the 110 people charged with a terrorist offence in 2017 have been prosecuted. All of them were convicted. A further 76 are awaiting prosecution. The remaining five cases have either not proceeded or are pending.

The latest figures show there were 224 terrorist prisoners in jails in Britain at the end of 2017 – an increase of 24%, or 43, over the previous year – and continuing the upward trend seen in recent years. The overwhelming majority – 86% – held Islamic extremist views while 9% were rightwing extremists including from the recently banned National Action,. The remaining 5% were classified as “other ideologies”.

The Home Office announced earlier this month that MI5 had reduced the threat level to Britain from Northern Ireland-related terrorism from substantial to moderate meaning a terrorist attack was possible but not likely. However, the threat to the UK from international terrorism remained at severe, meaning an attack was highly likely. The threat in Northern Ireland from Northern Irish terrorism also remains at severe.

The quarterly statistics show that the Metropolitan police has stepped up its use of counter-terror stop and search powers. The number of searches carried out rose 59% in 2017 to 767 from 483 the year before.

However, the number of people stopped and examined at ports and airports under counter-terror powers continued to fall in 2017. There were 16,349 examinations last year – a fall of 16% compared with the previous year, reflecting the recent downward trend. The number of people detained under these port powers, however, rose 10% in 2017 from 1,539 to 1,700.